1. Field of the Invention
The present invention relates to a method for removing particular metals from a feed stream, and more particularly for removing nickel and copper from a spent electroless deposition bath.
2. Description of the Prior Art
For a variety of technological reasons, electroless deposition of nickel and copper have come into widespread usage. These electroless baths, unlike more common electroplating baths, do not have a continuing life and are depleted as "spent baths" after a short period of time. Typically, after a very limited number of plating cycles (e.g., 5 to 10) the plating rate of an electroless bath will slow sufficiently to become marginal, and the bath has to be discarded as spent. A large percentage (e.g., about 50% of the nickel content) of the original formulation remains in the spent bath. For example, a nickel content of 10,000 ppm may be present in the spent electroless bath, a relatively high concentration as compared to, for example, the typical 1,000 ppm found in a spent rinse used to remove nickel plating solution from plated parts.
Since April, 1984, restrictions on the discharge of seven metals, including nickel and copper, have come into effect in the United States and the "dumping" of the spent baths is prohibited by law. In order to comply with the 1984 regulations, many users of electroless baths simply dosed their wastes with caustic soda to precipitate the metals as hydrous oxides, whereupon the hydrous oxides were filtered off, drummed, and sent off to landfills. Precipitation as a hydrous oxide is a technique which precipitates all but the alkali metals, but in a form which is highly unmanageable. A major chemical problem associated with such treatment develops from the presence of chelating agents in many electroless baths (such as citrates, lactates, glycine or EDTA), which elevate metal levels well above discharge limits at typical alkaline pH's. The addition of flocculants to spent baths to assist in the settling of the hydrous oxides often makes this mass "gummy" and difficult to manage. A major economic problem associated with such treatment is the need for a user to spend thousands of dollars in order to bury even more thousands of dollars worth of metal in landfills, only to achieve the dubious distinction of assuming perpetual liability for that portion of the landfill so occupied. However, as a result of the August, 1988 "landban" regulations of the Environmental Protection Agency (EPA) prohibiting the placement of metallic hydroxides (hydrous oxides) into landfills in the United States, even if alkaline precipitation methods were capable of being perfected, the disposal of the product in landfills is now precluded as a matter of law.
Notwithstanding the above problems associated with the removal of metals from a spent electroless bath as metallic hydroxides, the technique of precipitation to remove metallic compounds from a spent electroless bath remains sound. Precipitation followed by filtration is a preferred technique for metal removal, far less costly than the newer techniques of liquid-liquid extraction or ion exchange. When properly practiced, the technique of precipitation can yield the purest of crystalline products. The formation of crystals of a substance by precipitation from homogeneous solution is an exclusionary process; in order to exclude foreign matter from the product, it is usually only necessary to ensure that the precipitate is crystalline rather than amorphous.
Precipitation from homogeneous solution comprises a category of separation techniques which eliminates the problem caused by the mixing of the solution containing the material to be precipitated with the precipitating agent. (Upon adding the precipitating agent to the feed stream, a "zone of chaos" is temporarily created where the precipitate is poorly formed and often occludes all sorts of impurities.) Use of a precipitating agent which produces a metastable precipitation zone enables the formation of the precipitate to be delayed until the precipitating solution and the solution of the material to be precipitated become homogenous, so that the resultant formation of solids is orderly and exclusionary.
Homogeneous precipitation, as a practicable process, is usually achieved in one of two ways. In the first of these two ways, the precipitating agent is generated in situ by a chemical reaction--that is, the precipitating agent appears gradually from a precursor by a chemical reaction. For example, precipitation of barium sulfate by the addition of a sulfate ester to a barium solution can be effected in this manner. The purity and crystal size of barium sulfate so generated are greatly enhanced. In the second of these two ways, a chemical system is selected to have certain metastable solubility characteristics--that is, the difference in the value of the supersaturation and saturation limits at a given temperature for a given substance (that is, the so-called "metastable" region, a region where precipitation will eventually take place but only with the passage of time) is selected so that the period of time during which the metastable condition exists is considerable so that precipitation is from a homogeneous solution. See Santhanam et al., U.S. Pat. No. 4,278,539 for an example of such a system. Systems where the metastable region are large are usually characterized by having orientational requirements for crystal formation, whereas those systems where the metastable regions are small lack these orientational requirements. The hydrous oxides which are conventionally generated to remove metals from solution have little or no metastable regions and so precipitation is rapid and chaotic, and the precipitate is impure and difficult-to-harvest.
The chemistry of the metal oxalates has received much attention historically in analytic and inorganic chemistry, e.g., the classical method of separation of the lanthanides (rare earth elements) is based on the fractional crystallization of the double oxalates, and the classical simultaneous determination of calcium and magnesium by gravimetric means is as oxalates. Oxalates of such widely divergent metals as Tl, Pb, Ca, Be, Al, Cr, Ti, V, and Mn are well known. It has been reported that all nonalkali metals excepting beryllium form insoluble oxalates. Jordan, U.S. Pat. No. 4,018,876 relates to oxalate precipitation in alkaline media.
The analytical chemical approach to the precipitation of metal oxalates is to treat it as simple insoluble salt formation following the stoichiometric relationship: EQU mM.sup.n+ +nA.sup.m- =M.sub.m A.sub.n [Equation 1]
For oxalate precipitation, the following equation applies: EQU 2M.sup.n+ +C.sub.2 O.sub.4.sup.2- =CaC.sub.2 O.sub.4 [Equation 3]
For example, with calcium oxalate, the Equation 2 would be: EQU Ca.sup.2+ +C.sub.2 O.sub.4.sup.2- =M.sub.2 (Cphd 2O.sub.4)n [Equation 3]
The solubility product equation for Equation 3 is: [Ca.sup.2+ ][C.sub.2 O.sub.4.sup.2- ]=K.sub.sp =2.3.times.10.sup.-9 [Equation 4]
The effect of pH on the equilibrium ionization of oxalic acid is that the addition of acid will suppress the concentration of free oxalate ion and therefore increase the solubility of the metal. Learned treatises and scientific literature in this area specifically indicate that all the insoluble metal oxalates are soluble in mineral acid, owing to the removal of free oxalate ion. For example, it has been reported that "all oxalates are soluble in mineral acid." Treadwell and Hall, Analytical Chemistry, Vol. 1, p. 383 (6th Ed., John Wiley & Sons). Cadmium and lead, both of which have oxalates that are more insoluble than those of nickel and copper, apparently do not form insoluble oxalates under conditions of hyperacidity (The oxalate pK.sub.sp 's are Ni 7.0; Cu 7.5; Cd 7.8; and Pb 11.1). Insoluble calcium oxalate has been shown to resolubilize in acid solution as well. See Hearon et al., U.S. Pat. No. 3,998,878.
In Feld et al., U.S. Pat. No. 4,490,297, cobalt oxalate is said to be sparingly soluble in hydrobromic acid. The presumption is that acid will dissolve some cobalt oxalate. Wood, Jr., U.S. Pat. No. 4,246,185 recognizes that copper can be precipitated as an oxalate from a moderately acidic solution. Precipitation from a hyperacidic solution, however, is not contemplated by the teachings of the Wood, Jr. patent. Oxalic acid has heretofore not been considered as suitable for use as a precipitant of the metals to be removed from an acidic solution such as a spent electroless bath.
Accordingly, it is an object of the present invention to provide a method of processing an aqueous feed liquid (such as a spent electroless bath) to precipitate a metal therein in a form suitable for filtration so as to leave the feed liquid suitable for discharge to a sewer line.
Another object is to provide such a method which precipitates the metal in a purified form without the occlusion of impurities.
A further object is to provide such a method which precipitates from the feed liquid any nickel and copper metals dissolved therein.
It is also an object of the present invention to provide such a method which facilitates the separation of the two metals in a feed liquid both from the feed liquid and each other.
It is another object to provide a bath suitable for use in such a method.